Autonomous underwater vehicle propulsion systems typically use electric motors with mechanical gearing to reduce the rotational speed, such that it is well matched to the peak operating point of a propeller [see, for e.g., B. Claus, R. Bachmayer, and C. D. Williams, “Development of an auxiliary propulsion module for an autonomous underwater glider,” Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment, vol. 224, no. 4, pp. 255-266, 2010. [Online]. Available: http://pim.sagepub.com/content/224/4/255.abstract; and M. E. Furlong, D. Paxton, and P. Stevenson, “Autosub long range: A long range deep diving AUV for ocean monitoring,” in Autonomous Underwater Vehicles (AUV), 2012 IEEE/OES, September 2012, pp. 1-7]. Alternative designs use a custom built direct drive brushless motor for reliability reasons but unfortunately they have higher losses and are significantly larger than their geared counterparts [see, for e.g., J. Bellingham, Y. Zhang, J. Kerwin, J. Erikson, B. Hobson, B. Kieft, M. Godin, R. McEwen, T. Hoover, J. Paul, A. Hamilton, J. Franklin, and A. Banka, “Efficient propulsion for the Tethys long-range autonomous underwater vehicle,” in Autonomous Underwater Vehicles (AUV), 2010 IEEE/OES, September 2010, pp. 1-7]. A magnetically coupled gearing system, however, has several significant advantages compared to classical mechanical gear reduction methods. For one, the lack of physical contact between rotating parts—other than engineered bearing surfaces—eliminates excessive mechanical wear and greatly reduces the effect of load disturbances. In addition, inherent overload protection resulting from the physical isolation of input and output rotors, prevents premature failure of components in the event of a sudden torque application. The lack of mechanical contact also leads to a reduction in vibrations and acoustic noise. Furthermore, a magnetic gear can eliminate the need for rotary shaft seals by isolating the motor and drive electronics from the environment with a barrier in the air gap. As a result, one can appreciate that a magnetic gear system is well suited for underwater vehicle propulsion systems where long term, maintenance-free application is an asset. Magnetic gearing itself has been the subject of fascination for well over one hundred years with patents on the subject first showing up as early as 1913 [see, for e.g., A. H. Neuland, “Apparatus for transmitting power,” U.S. Pat. No. 1,171,351, 1916]. These early gears, however, were often complex machines with low torque density and did not see wide spread use [see, for e.g., H. T. Faus, “Magnet gearing,” U.S. Pat. No. 2,243,555, 1941; K. Tsurumoto and S. Kikuchi, “A new magnetic gear using permanent magnet,” Magnetics, IEEE Transactions on, vol. 23, no. 5, pp. 3622-3624, September 1987; and S. Kikuchi and K. Tsurumoto, “Design and characteristics of a new magnetic worm gear using permanent magnet,” Magnetics, IEEE Transactions on, vol. 29, no. 6, pp. 2923-2925, November 1993]. More recently, in 2001 Atallah et al. presented an initial concept for a high torque density magnetic gear comprising three rotors, namely a high speed, low speed and modulating rotor containing ferromagnetic pole pieces [see K. Atallah and D. Howe, “A novel high-performance magnetic gear,” Magnetics, IEEE Transactions on, vol. 37, no. 4, pp. 2844-2846, July 2001]. A follow up paper presented the experimental validation in which torque densities of up to 100 kNm/m3 with up to 97% efficiency and gear ratios of 2-12 were demonstrated [see K. Atallah, S. Calverley, and D. Howe, “Design, analysis and realisation of a high-performance magnetic gear,” Electric Power Applications, IEEE Proceedings—, vol. 151, no. 2, pp. 135-143, March 2004].
What is needed, however, is a magnetically geared electric drive having a design that is intended and well suited for reliable, long term application in an underwater vehicle where efficiency of energy transmission is useful.